Subsurface well tools are of many types. One type is actuated by an explosive or combustible charge. These tools have a variety of functions, including such different devices as perforating guns, formation fracturing devices shown in U.S. Pat. Nos. 4,530,396; 4,823,876; 4,852,647; 4,976,318 and 5,005,641, well cleaning devices shown in U.S. Pat. No. 4,976,318, and the like. Typically, these devices are actuated by an electrical signal delivered to the subsurface well tool through an insulated cable. The electrical signal normally initiates combustion of a small powder charge or detonates a blasting cap. As used herein, the words combustion and combustible are intended to be broad enough to encompass explosion and explosive because the difference between the processes is essentially the rate at which the chemical combination of fuel and oxidant occurs.
There are a number of situations where it is necessary or desirable to initiate operation of subsurface combustible well tools by hydraulic or mechanical techniques because it is impractical or undesirable to run the tool on a wireline. An early example of mechanical actuation of a subsurface combustible well tool is a perforating gun. It is often desirable to run a perforating gun on the bottom of a tubing string so the well can be perforated with large, high capacity shaped charges in an underbalanced situation, i.e. where the tubing string is essentially empty so the formation pressure is much greater than the hydrostatic pressure inside the tubing string.
Ordinary wire line operations are awkward in wells with long horizontal sections because it is difficult to get the wireline tool into the horizontal section. In addition, dropped go-devils will not mechanically actuate a detonator because it will slow down or stop in the horizontal section. Thus, the only feasible technique of firing a perforating gun in a horizontal well is with a pressure actuated firing head.
It is also desirable to initiate operations of subsurface combustible well tools by hydraulic or mechanical means where liquids or liquid-gas mixtures are being pumped into a well. For example, sand consolidation operations are conducted where a propellant charge is ignited to provide a high pressure gas to drive the uncured resin from the well bore through the perforation tunnels and into the formation. A curing agent is then pumped into the zone to cure the resin. While such an operation may be conducted with a small diameter coiled tubing work string inside a production tubing string and an electric wireline cable inside the coiled tubing work string, it is often impractical because the wireline restricts flow through the coiled tubing. In addition, there is a risk of lodging the tool inside the coiled tubing if the resin becomes more viscous due to causes such as spontaneous polymerization. It is accordingly desirable to have the option of igniting the propellant charge using mechanical or hydraulic means rather than electric wireline.
The advantage of mechanically or hydraulically initiating operation of a combustible subsurface tool occurs in other multiple stage operations where it liquids or gases are injected into the formation. These operations include the injection of clay stabilizers, scale inhibitors, solvents such as xylene, acids and the like. These materials may be injected into the formation before or after ignition of the combustible device.
The standard technique for initiating operation of a combustible subsurface tool is by dropping a projectile, such as a ball, bar or go-devil through an empty tubing string. Because the projectile falls through air, it reaches a substantial velocity and has sufficient energy to break a shear pin, drive home a firing pin or otherwise set off the combustible igniter. Typical disclosures are found in U.S. Pat. Nos. 2,254,979; 2,067,408; 2,760,408 and 4,658,900.
Where the tubing string is full of liquid, dropped projectiles do not work because they do not reach substantial velocities and therefore do not generate large forces that may be necessary or desirable to initiate operation. Similar problems occur in horizontal well sections because the dropped bars or go-devils slow down or stop in the horizonal well sections.
Another disclosure of interest is found in U.S. Pat. No. 4,260,017.